The present invention relates to a control system for a work machine and a method for controlling at least one hydraulic cylinder.
The invention will be described below in connection with a work machine in the form of a wheel loader. This is a preferred but in no way limiting application of the invention. The invention can also be used for other types of work machines (or work vehicles), such as an excavator loader (backhoe) and excavating machine.
The invention relates, for example, to controlling lifting and/or tilting cylinders for operating an implement.
More precisely, the invention relates to a control system which comprises a hydraulic machine which functions as both pump and motor. The hydraulic machine is connected in a driving manner to an electric machine which functions as both motor and generator.
The hydraulic machine therefore functions as a pump in a first operating state and supplies pressurized hydraulic fluid to the hydraulic cylinder. The hydraulic machine also functions as a hydraulic motor in a second operating state and is driven by a hydraulic fluid flow from the hydraulic cylinder. The electric machine therefore functions as an electric motor in the first operating state and as a generator in the second operating state.
The first operating state corresponds to a work operation, such as lifting or tilting, being carried out with the hydraulic cylinder. Hydraulic fluid is therefore directed to the hydraulic cylinder for movement of the piston of the cylinder. On the other hand, the second operating state is an energy recovery state.
A first object of the invention is to provide a control system, preferably for a lifting and/or tilting function, which affords an opportunity for energy-efficient operation.
According to an aspect of the present invention, a control system is provided for a work machine, which system comprises an electric machine, a hydraulic machine and at least one hydraulic cylinder, the electric machine being connected in a driving manner to the hydraulic machine, the hydraulic machine being connected to a piston side of the hydraulic cylinder via a first line and a piston-rod side of the hydraulic cylinder via a second line, the hydraulic machine being adapted to be driven by the electric machine and supply the hydraulic cylinder with pressurized hydraulic fluid from a tank in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder and drive the electric machine in a second operating state.
The hydraulic cylinder is preferably adapted to move an implement in order to perform a work function. According to a first example, the hydraulic cylinder comprises a lifting cylinder for moving a loading arm which is pivotably connected to a vehicle frame, the implement being arranged on the loading arm. According to a second example, the hydraulic cylinder comprises a tilting cylinder for moving the implement which is pivotably connected to the loading arm.
The speed of the cylinder is preferably controlled directly by the electric machine, that is to say no control valves are required between the hydraulic machine and the cylinder for regulating direction and speed of the movement. In some cases, on/off valves which open and respectively close a communication for the hydraulic fluid flow are required.
According to a preferred embodiment of the invention, the hydraulic machine has a first port which is connected to the piston side of the hydraulic cylinder via the first line and a second port which is connected to the piston-rod side of the hydraulic cylinder via the second line. The second port is thus separated from the first port. In addition, the hydraulic machine is preferably arranged to be driven in two different directions, with one direction being associated with a flow out from the first port and the second direction being associated with a flow out from the second port. The hydraulic machine is thus capable of pumping in both directions.
According to another preferred embodiment of the invention, the system comprises a means for controlling pressure, which pressure means is arranged on a line between the hydraulic machine and the tank, to achieve a pressure build-up between the hydraulic machine and the pressure means. In this way, it is possible to achieve a refilling of the piston-rod side of the hydraulic cylinder during lowering, a forced lowering of the implement (so-called “power down”), and additional energy recovery, etc. Either the piston side or the piston-rod side is preferably connected to the line between the hydraulic machine and the pressure means.
A second object of the invention is to achieve a method for controlling a hydraulic cylinder that makes it possible to carry out a forced lowering (“power down”) of the implement.
According to an aspect of the present invention, a method is provided for controlling a hydraulic cylinder under the influence of a load, with a hydraulic machine being operatively connected to the hydraulic cylinder via a first line and to a tank via a second line, comprising the steps of controlling the hydraulic machine in such a way that it is allowed to be driven by a flow of hydraulic fluid from the hydraulic cylinder, of detecting an operating parameter that is indicative of a pressure on the piston side of the hydraulic cylinder, of comparing the detected pressure with a predetermined level and of increasing the pressure on the piston-rod side of the hydraulic cylinder if the detected pressure is less than the predetermined level.
By this means, it is also possible to achieve a refilling of the piston-rod side of the hydraulic cylinder during lowering.
A third object of the invention is to achieve a method that makes possible an efficient recovery of energy during movement of the hydraulic cylinder under the influence of a load.
According to an aspect of the present invention, a method is provided for regeneration of energy during movement of a hydraulic cylinder under the influence of a load, with a hydraulic machine being operatively connected to the hydraulic cylinder via a first line and to a tank via a second line, comprising the steps of controlling the hydraulic machine in such a way that it is allowed to be driven by a flow of hydraulic fluid from the hydraulic cylinder, of detecting at least one operating parameter and of increasing the pressure in the line between the hydraulic machine and the tank, on the basis of the detected operating parameter, in order to increase the pressure on the tank side of the hydraulic machine.
Further preferred embodiments and advantages of the invention emerge from the following description.